Fuel cells are a leading alternate fuel power plant candidates for both portable and stationary electrical power generation. A fuel cell is an electrical chemical energy converter consisting of two electrodes which sandwich an electrolyte. In one form, an ion-conducting polymer electrolyte membrane (PEM) is disposed between two electrode layers to form a membrane electrode assembly (MEA). The MEA is typically porous and electrically conductive to promote the desired electrochemical reaction from two reactants. One reactant, oxygen or air, passes over one electrode and hydrogen, the other reactant, passes over the other electrode to produce electricity, water and heat. Typical PEM fuel cells with membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994 and assigned to the General Motors Corporation.
The hydrogen that is consumed by the fuel cell can be produced by a fuel processor that converts a hydrocarbon fuel, such as gasoline, methanol, or natural gas, into a hydrogen-containing reformate stream that can be used to make electricity in the fuel cell stack. The typical fuel processor uses three steps to produce such a conversion. In a first step, the primary reactor (usually an auto thermal reactor (ATR) or steam reformer (SR)) converts the hydrocarbon fuel to the hydrogen-containing reformate. However, during the first step significant levels of carbon monoxide (CO) may be present (6%-10%). Thus, in a second step, also known as a CO-reduction step, a water gas shift reactor (WGS) is typically used to reduce the CO content to about 0.3% to 1% and increase the hydrogen content of the hydrogen-containing reformate stream. In a third or final step, also known as a CO-polishing step, the CO content is further reduced to about 2 to 50 ppm depending on the fuel cell stack used. This final step is accomplished typically with either a preferential oxidation (PrOx) reactor or a pressure-swing absorption (PSA) device.
The hydrogen-containing reformate stream produced in the primary reactor exits the primary reactor at an elevated temperature. The reformate stream is typically cooled with either a fuel cell stack coolant or ambient air to reduce the temperature prior to entering the CO-reduction or CO-polishing stages. The heat extracted from the reformate stream is lost thermal energy. In some fuel processor systems, to maximize the fuel processor efficiency the heat extracted from cooling the reformate stream is integrated back into the system as heat required to generate and superheat steam used in the primary reactor and elsewhere in the system.
Previous attempts to recover heat from the cooled reformate have relied upon the use of an intermediate heat transfer fluid, such as oil or the like. The use of an intermediate heat transfer fluid, however, can increase the size of the fuel processor system along with increasing the complexity. Increasing the size and/or complexity of the fuel processor system may be undesirable, particularly in a portable application wherein space may be at a premium. Thus, it is desirable to provide a means of integrating the heat from cooling the reformate stream back into a fuel processor system utilizing a compact heat exchanger. Further, it would be advantageous if such can be performed while minimizing the complexity of such a system.
Each of the stages on the fuel processor system run at different reactor temperatures. The operation of these different reactors can be improved by supplying a reformate stream that has a generally uniform temperature profile. That is, the efficiency of the fuel processor system can be improved by proving a reformate stream that has a generally uniform temperature profile so that the downstream reactors can be designed to operate in a narrower range that conforms to the temperature profile of the reformate stream. Thus, it would be advantageous to produce a reformate stream having a generally uniform temperature profile while extracting heat from the reformate stream.
Furthermore, the use of a heat exchanger to remove heat from the reformate stream can result in a pressure drop and/or a change in a velocity profile of the reformate stream. The pressure drop and velocity changes represent lost energy that, if minimized, can increase the efficiency of the fuel processor system. Thus, it would be advantageous to extract heat from the reformate stream while providing a minimal pressure drop and/or providing a generally uniform velocity profile of the reformate stream.